Implements passing access rights (i.e. file descritors) over Unix domain sockets. Only one fd can be passed at one time and no other data can be sent in the same operation (operation itself involves sending exactly one byte of data to solve EOF detection anomaly).

Specifically, the code from [Stevens] did not work out of the box. I had to rename msg.msg_accrights* to msg.control* and send at least one byte. General impression from net is that file descripto passing code seems to be buggy - not just in Linux, but on FreeBSD, too.